1. Field of the Invention
The present invention generally relates to a metal/ceramic bonding substrate and a method for producing the same. More specifically, the invention relates to a metal/ceramic bonding substrate for use in super power electronic parts, such as power modules, a method for producing the same, and a power module using the metal/ceramic bonding substrate.
2. Description of the Prior Art
In recent years, power modules are used for controlling heavy-current for electric vehicles, electric railcars and machine tools. In conventional power modules, a ceramic insulating substrate is fixed by soldering to one side of a metal plate or compound material called base plate, and power ICs are fixed by soldering to the ceramic insulating substrate. On the other side (which will be hereinafter referred to as a “reverse”) of the base plate, a radiating fin or cooling jacket of a metal is mounted via a thermal conduction grease by means of screws.
Since the soldering of the base plate and power ICs on the ceramic insulating substrate is carried out by heating, the base plate is easy to warp due to the differential coefficient of thermal expansion between bonding members during soldering. Heat generated by the power ICs passes through the ceramic insulating substrate, solder and base plate to be radiated from the radiating fin or cooling jacket to air or cooling water. Therefore, if the base plate warps during soldering, clearance increases when the radiating fin or cooling jacket is mounted on the base plate, so that there is a problem in that the heat sink characteristic of the substrate extremely deteriorates.
The reason for this is as follows. Each of the base plate, radiating fin and cooling jacket is made of a metal, and warps to some extent. Therefore, even if the base plate is caused to contact the radiating fin or cooling jacket, there are non-contact portions therebetween, so that it is not possible to obtain efficient heat conduction. In order to compensate for this, a thermal conduction grease is applied on the base plate and/or the radiating fin or cooling jacket to cause the radiating fin or cooling jacket to tightly contact the base plate. However, the thermal conductivity of the thermal conduction grease is a few W/m which is far smaller than that of other members, such as the base plate. Thus, in order to prevent heat sink characteristic from deteriorating due to the thermal conduction grease, the thermal conduction grease having the minimum thickness of tens to hundreds micrometers necessary to compensate for the fine warpage of the base plate or the like is usually provided between the base plate and the radiating fin or cooling jacket. However, if only the base plate warps by one hundred micrometers, the thickness of that portion of the thermal conduction grease increases by one hundred micrometers, so that heat sink characteristic deteriorates.
In particular, if the reverse of the base plate greatly warps so as to be concave, it is a deathblow, and the thickness of the thermal conduction grease on the central portion of the base plate increases. In order to avoid this, the base plate is usually caused to previously warp by a mechanical operation, and a side of the base plate on which the radiating fin or cooling jacket is to be mounted is caused to warp so as to be flat or convex.
Recently, there has been provided a metal/ceramic circuit board wherein a base plate of aluminum or an aluminum alloy is bonded directly to a ceramic insulating substrate without the need of soldering (see, e.g., Japanese Patent Laid-Open No. 2002-76551).
In the case of the metal/ceramic circuit board disclosed in Japanese Patent Laid-Open No. 2002-76551, it is not required to solder the base plate on the ceramic insulating substrate, so that thermal conductivity is improved therebetween. However, it is required to solder power ICs on the circuit, so that it is required to cause the reverse of the base plate to warp so as to be as flat or convex as possible after heating.
In order to produce the metal/ceramic circuit board disclosed in Japanese Patent Laid-Open No. 2002-76551, molten aluminum is poured onto the ceramic substrate in a shape corresponding to that of the base plate, and solidified to bond an aluminum plate to the ceramic substrate. When such a metal/ceramic circuit board is caused to pass through a furnace in order to solder power ICs, if the base plate and the ceramic substrate contacting therewith are large, the reverse of the base plate tends to be easy to greatly warp so as to be concave. Thus, if the reverse of the base plate greatly warps so as to be concave, there is a problem in that the heat sink characteristic of the board deteriorates due to bad adhesion between the heat sink plate and the radiating fin or cooling jacket.